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Ultrasound pictures are formed using sound waves. A machine sends sound waves through the body, which are then reflected back and converted into an image that is visible on a screen. Ultrasound has revolutionised the care of women during pregnancy.
In the UK an ultrasound scan is a routine examination, usually performed when a woman first attends the antenatal clinic and often again at 18 to 22 weeks of pregnancy.
How is ultrasound scanning carried out?
In the first 10 weeks of pregnancy the best way to scan is by inserting a small probe into the vagina. The examination is similar to a pelvic examination. Even embryos that are only a few millimetres long will be visible on the TV monitor.
In later stages of the pregnancy, the scanning will be done via the surface of the abdomen. Ultrasound gel will be spread on the skin, then the scanner is passed over the uterus until the foetus and the placenta are found. Usually, the pregnant woman and her partner can watch the scan on the monitor.
What is ultrasound scanning used for?
Ultrasound is used for different reasons at different stages in pregnancy. In the first stage of the pregnancy, usually before 14 weeks, Ultrasound scanning is used to check whether the foetus is alive and whether it is alone or one of twins or triplets.
By measuring the length of the foetus it is also possible to accurately determine when the baby will be due.
Some major abnormalities can also be detected at this stage. At 11 to 14 weeks, measurement of the thickness of the skin at the back of the neck (known as nuchal translucency measurement) can be used to calculate the risk of the foetus having a chromosome abnormality.
From 18 weeks onwards, it is possible to examine the foetus in more detail. Most organ systems can be examined to ensure that the foetus appears to be developing normally. The spine, skull, brain, heart, lungs, kidneys, arms and legs can all be seen. If the mother is overweight, then the quality of the examination may be poor.
From 30 weeks onwards, Ultrasound is often used to estimate how well the baby appears to be growing. It is difficult to be precise about this but it is often useful if the woman has had a small baby in the past or has a condition that may affect the baby's growth, such as pre-eclampsia. The bloodstream in the umbilical cord is also examined to see if it is functioning well enough to transport sufficient oxygen and nutrition to the foetus.
It is also possible to check the position of the placenta to see whether it is lying normally or if the placenta is lying abnormally close to the inside of the cervix (a condition known as placenta praevia).
Will an ultrasound scan guarantee a healthy child?
An Ultrasound scan can provide a good overall impression of the foetus, but even the finest examination cannot detect all possible problems.
Can ultrasound scans harm the unborn child?
There is no scientific evidence to support this concern.
What if the scan shows a problem?
You will naturally be very worried if your scan suggests that there is a problem with your baby. Sometimes a definite diagnosis can be made from the scan, such as spina bifida. In other cases the scan may show minor changes, known as "markers", which may be a sign of a more serious problem, such as Down's syndrome, or may turn out to be just a variation of normal.
If the sonographer finds anything unusual when doing the scan, she should refer you to a doctor within 24 hours and, if it is necessary, to a Fetal Medicine Specialist within 72 hours. It may be necessary for the doctor to perform a further test such as a CVS or amniocentesis to see if the baby's chromosomes are normal.
In the unlikely event that your baby does have a serious health problem, you will need time and support to think through your choices. These may include ending the pregnancy, or preparing for the birth of a baby who needs special care or, in rare cases, carrying out surgery on your unborn baby. If you find yourself in this situation and are looking for support and information, contact the charity Antenatal Results and Choices (ARC).
Is ultrasound safe?
Ultrasound has been used in pregnancy for nearly 30 years and medical research has found no side effects. No association has been shown between Ultrasound exposure and the baby's birthweight, childhood leukaemias or other cancers, eyesight, hearing or dyslexia. Even so, most experts agree that the procedure shouldn't be done without clear medical reasons, and that all Ultrasound exposure should be justified and limited to the minimum needed to make a diagnosis.
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